Sign into the Mac Dev centre and download the GM seed, Mount the .DMG then copy the .App to your Applications folder. Run the app to install just like when you upgraded from Lion to ML etc etc
[You do not have to upgrade all the way to B8, you can install GM right over it]
That's not quite how it works. Despite there being a DMG floating around the net at the moment, that's not what apple provide.
If you want to get it the legitimate way, this is how:
In the mac dec center, you get a download code, which you then redeem in the mac app store. This then downloads the installer file directly to your applications folder. There is no DMG involved.